Long range rfid device for battery monitoring and systems implementing same

ABSTRACT

An RFID system according to one embodiment includes an electronic device being powered by a battery; an RFID device in electrical communication with the electronic device; and a mechanism for estimating a remaining potential energy of the battery, wherein a flag is set on the RFID device when an estimated remaining potential energy of the battery is below a predefined threshold. In an RFID system according to another embodiment, the RFID device stores an indication of a condition of the battery powering the electronic device. An RFID device according to yet another embodiment of the invention includes an interface for providing a direct physical connection to an electronic device that is powered by a battery; a memory for storing an indication of a condition of the battery powering the electronic device; and circuitry for sending the indication stored in the memory to a remote device via an air interface.

FIELD OF THE INVENTION

The present invention relates to Radio Frequency Identification (RFID)systems and methods, and more particularly, this invention relates tosystems and methods using RFID devices for indicating a condition of abattery powering peripheral electronic devices.

BACKGROUND OF THE INVENTION

The use of RFID tags are quickly gaining popularity for use in themonitoring and tracking of an item. RFID technology allows a user toremotely store and retrieve data in connection with an item utilizing asmall, unobtrusive tag. As an RFID tag operates in the radio frequency(RF) portion of the electromagnetic spectrum, an electromagnetic orelectrostatic coupling can occur between an RFID tag affixed to an itemand an RFID tag reader. This coupling is advantageous, as it precludesthe need for a direct contact or line of sight connection between thetag and the reader.

Addition of supplemental power to RFID tags, e.g., from a battery, hasgreatly increased the range in which reliable communication with the tagis possible. This has in turn made new applications possible.

One concern with self-powered electronic devices is the life of thebattery. Battery-powered devices draw very little power when inactive orturned off, but draw orders of magnitude more power when active. If theelectronic device has been activated many times, the battery will beused up more quickly than for a device activated less. Because certainelectronic devices are active more often than others, depending upon theduty cycle of use and/or may simply draw more power than other devices,it is hard to estimate the battery life of a given electronic device. Ina situation where there are many electronic devices, the current methodis to replace all batteries when the battery on one of the electronicdevices dies, as it is likely others will die soon as well. However, itis quite possible that many of the batteries may still have a reasonableuseful operating life remaining. Thus it would be desirable to providean indication of the condition of a battery of an individual electronicdevice.

SUMMARY OF THE INVENTION

A Radio Frequency Identification (RFID) system according to oneembodiment of the present invention includes an electronic device beingpowered by a battery; an RFID device in electrical communication withthe electronic device; and a mechanism for estimating a remainingpotential energy of the battery, wherein a flag is set on the RFIDdevice when an estimated remaining potential energy of the battery isbelow a predefined threshold.

An RFID system according to another embodiment of the present inventionincludes an electronic device being powered by a battery; and an RFIDdevice in electrical communication with the electronic device, whereinthe RFID device stores an indication of a condition of the batterypowering the electronic device.

An RFID device according to yet another embodiment of the presentinvention includes an interface for providing a direct physicalconnection to an electronic device that is powered by a battery; amemory for storing an indication of a condition of the battery poweringthe electronic device; and circuitry for sending the indication storedin the memory to a remote device via an air interface.

A method for indicating a condition of a battery according to a furtherembodiment of the present invention includes estimating a remainingpotential energy of a battery coupled to an electronic device; setting aflag on an RFID device when the estimated remaining potential energy ofthe battery is below a predefined threshold; and sending a state of theflag to a remote device via an air interface.

Other aspects and advantages of the present invention will becomeapparent from the following detailed description, which, when taken inconjunction with the drawings, illustrate by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and advantages of the presentinvention, as well as the preferred mode of use, reference should bemade to the following detailed description read in conjunction with theaccompanying drawings.

FIG. 1 is a system diagram of an RFID system.

FIG. 2 is a system diagram for an integrated circuit (IC) chip forimplementation in an RFID device.

FIG. 3 is a system diagram of an RFID device according to one embodimentof the present invention.

FIG. 4A is a system diagram of an electronic device in which acontroller, e.g., chip, of an RFID device is located on a same printedcircuit board as a controller of the electronic device.

FIG. 4B is a system diagram of an electronic device and an RFID deviceexternal thereto.

FIG. 5 is a process diagram of a method for communication between anelectronic device and a remote device according to one embodiment of thepresent invention.

FIG. 6 is a process diagram of a method for communication between anelectronic device and a remote device according to one embodiment of thepresent invention.

FIG. 7 is a system diagram of an electronic device with sensorcapability and an optional display device according to one embodiment ofthe present invention.

FIG. 8 is a system diagram of an electronic display device according toone embodiment of the present invention.

FIG. 9 is a process diagram of a method for updating an RFID-baseddisplay according to one embodiment of the present invention.

FIG. 10 is a system diagram of a system that provides intermittentbattery monitoring.

FIG. 11 is a circuit diagram of a circuit for continuous batterymonitoring.

FIG. 12 is a circuit diagram of a circuit for continuous batterymonitoring with automatic calibration.

FIG. 13A is a top view of an exemplary Ultra-thin Quad Flat No-Lead(UQFN) package that may be used in conjunction with an illustrativeembodiment.

FIG. 13B is a side view of the package of FIG. 13A, taken along line13B-13B of FIG. 13A.

FIG. 13B is a bottom view of the package of FIG. 13A, taken along line13C-13C of FIG. 13B.

BEST MODE FOR CARRYING OUT THE INVENTION

The following description is the best mode presently contemplated forcarrying out the present invention. This description is made for thepurpose of illustrating the general principles of the present inventionand is not meant to limit the inventive concepts claimed herein.Further, particular features described herein can be used in combinationwith other described features to create a plethora of various possiblecombinations and permutations.

Unless otherwise specifically defined herein, all terms are to be giventheir broadest possible interpretation including meanings implied fromthe specification as well as meanings understood by those skilled in theart and as defined in dictionaries, treatises, etc.

FIG. 1 depicts an RFID system 100 according to one of the variousembodiments, which may include some or all of the following componentsand/or other components. As shown in FIG. 1, one or more RFID devices102 are present. Each RFID device 102 in this embodiment includes acontroller and memory, which are preferably embodied on a single chip asdescribed below, but may also or alternatively include an applicationspecific integrated circuit (ASIC), external memory module, etc. Forpurposes of the present discussion, the RFID devices 102 will bedescribed as including a chip. An illustrative chip is disclosed below,though actual implementations may vary depending on how the device is tobe used. In general terms, a preferred chip includes one or more of apower supply circuit to extract and regulate power from the RF readersignal; a detector to decode signals from the reader; a backscattermodulator, a transmitter to send data back to the reader; anti-collisionprotocol circuits; and at least enough memory to store its uniqueidentification code, e.g., Electronic Product Code (EPC).

Each RFID device 102 may further include or be coupled to an antenna.

While RFID devices 102 according to some embodiments are functional RFIDtags, other types of RFID devices 102 include merely a controller withon-board memory, a controller and external memory, etc.

Each of the RFID devices 102 may be coupled to an object or item, suchas an article of manufacture, a device, a person, etc. According to apreferred embodiment, at least one RFID device 102 is in electricalcommunication with an electronic device 103 via a direct physicalconnection, e.g., via a bus, wires, etc.

The electronic device 103 can be any type of electronic device,including but not limited to monitoring equipment such as sensors,anti-theft devices, graphical and textual display devices, printers,etc.

In one aspect, data (e.g., information, instructions, etc.) received bythe RFID device from a remote device 104 is stored in the memory of theRFID device 102, the data being communicated to the electronic device103 via the direct physical, connection. In another aspect, datainitiating with the electronic device 103 is stored in the memory of theRFID device 102 for subsequent transmission to the remote device.Examples of communication between the devices are presented below.

The electronic device 103 may include a memory for storing data receivedfrom the RFID device memory.

As alluded to above, the direct physical connection can be any type ofphysical connection, and preferably allows bidirectional communication,but need not in all embodiments. Illustrative direct physicalconnections include one or more wires coupling the devices, a plug andsocket arrangement, connections on a common printed circuit board, etc.In a preferred embodiment, a bus such as an industry-standard I²C or SPIinterface is appended to the RFID device chip, with a complementary buson the electronic device 103 positionable in contact with the RFIDdevice bus, thereby allowing the electronic device 103 to communicatedirectly with the chip of the RFID device 102.

In some embodiments, the RFID device 102 is not mounted to theelectronic device 103. In other embodiments, the RFID device 102 ispermanently or detachably mounted on the electronic device 103. Inanother aspect, the RFID device 102 is physically integrated with theelectronic device 103. For example, FIG. 4A depicts an embodiment wherethe controller 402, e.g., chip, of the RFID device 102 is located on asame printed circuit board 404 as a controller 406 of the electronicdevice 103. In another example, the RFID device 102 may be positionedwithin a housing of the electronic device 103. FIG. 4B illustrates anembodiment where the RFID device 102 is external to the electronicdevice 103.

With continued reference to FIG. 1, a remote device 104 such as aninterrogator or “reader” communicates with the RFID devices 102 via anair interface, preferably using standard RFID protocols. An “airinterface” refers to any type of wireless communications mechanism, suchas the radio-frequency signal between the RFID device and the remotedevice. The RFID device 102 executes the computer commands that the RFIDdevice 102 receives from the reader 104 and/or electronic device 103.

The system 100 may also include an optional server 106 or other backendsystem which may include databases containing information and/orinstructions relating to RFID tags and/or tagged items.

In a preferred embodiment, a long-range RFID tag is electrically coupledto an external device, and acts as a means of communicating data betweenthe external device and an RFID reader. The external device can be asensor, display, printer, etc. Data, instructions, etc. can be uploadedfrom the reader to the external device via the tag. For example, wherethe external device is a display, updated display data can be uploadedto the tag for download (via digital bus) and output on the externaldevice.

Likewise, data can be downloaded from the external device to a readervia the tag. Thus, where the external device is a sensor, readings canbe periodically downloaded.

Preferably, the RFID device 102 has sufficient memory to store the data,instructions, etc. so that the external device itself need not be inactive communication with the remote device. Rather, the instructionsare stored locally on the RFID device 102, for later download by theelectronic device 103 coupled thereto.

As noted above, each RFID device 102 may be associated with a uniqueidentifier. Such identifier is preferably an EPC code. The EPC is asimple, compact identifier that uniquely identifies objects (items,cases, pallets, locations, etc.) in the supply chain. The EPC is builtaround a basic hierarchical idea that can be used to express a widevariety of different, existing numbering systems, like the EAN.UCCSystem Keys, UTD, VIN, and other numbering systems. Like many currentnumbering schemes used in commerce, the EPC is divided into numbers thatidentify the manufacturer and product type. In addition, the EPC uses anextra set of digits, a serial number, to identify unique items. Atypical EPC number contains:

-   -   1. Header, which identifies the length, type, structure, version        and generation of EPC;    -   2. Manager Number, which identifies the company or company        entity;    -   3. Object Class, similar to a stock keeping unit or SKU; and    -   4. Serial Number, which is the specific instance of the Object        Class being tagged.

Additional fields may also be used as part of the EPC in order toproperly encode and decode information from different numbering systemsinto their native (human-readable) forms.

Each RFID device 102 may also store information about the item to whichcoupled, including but not limited to a name or type of item, serialnumber of the item, date of manufacture, place of manufacture, owneridentification, origin and/or destination information, expiration date,composition, information relating to or assigned by governmentalagencies and regulations, etc. Furthermore, data relating to an item canbe stored in one or more databases linked to the RFID tag. Thesedatabases do not reside on the tag, but rather are linked to the tagthrough a unique identifier(s) or reference key(s).

RFID systems may use reflected or “backscattered” radio frequency (RF)waves to transmit information from the RFID device 102 to the remotedevice 104, e.g., reader Since passive (Class-1 and Class-2) tags getall of their power from the reader signal, the tags are only poweredwhen in the beam of the reader 104.

The Auto ID Center EPC-Compliant tag classes are set forth below:

Class-1

-   -   Identity tags (RF user programmable, range ˜3 m)    -   Lowest cost

Class-2

-   -   Memory tags (20 bit address space programmable at ˜3 m range)    -   Security & privacy protection    -   Low cost

Class-3

-   -   Semi-passive tags (also called semi-active tags and battery        assisted passive (BAP) tags)    -   Battery tags (256 bits to 2M words)    -   Self-Powered Backscatter (internal clock, sensor interface        support)    -   ˜100 meter range    -   Moderate cost

Class-4

-   -   Active tags    -   Active transmission (permits tag-speaks-first operating modes)    -   ˜30,000 meter range    -   Higher cost

In RFID systems where passive receivers (i.e., Class-1 and Class-2 tags)are able to capture enough energy from the transmitted RF to power thedevice, no batteries are necessary. In systems where distance preventspowering a device in this manner, an alternative power source must beused. For these “alternate” systems (also known as semi-active,semi-passive or battery-assisted), batteries are the most common form ofpower. This greatly increases read range, and the reliability of tagreads, because the tag does not need power from the reader to respond.Class-3 tags only need a 5 mV signal from the reader in comparison tothe 500 mV that Class-1 and Class-2 tags typically need to operate. This100:1 reduction in power requirement along with the reader's ability tosense a very small backscattered signal permits Class-3 tags to operateout to a free space distance of 100 meters or more compared with aClass-1 range of only about 3 meters. Note that semi-passive and activetags with built in passive mode may also operate in passive mode, usingonly energy captured from an incoming RF signal to operate and respond,at a shorter distance up to 3 meters.

Active, semi-passive and passive RFID tags may operate within variousregions of the radio frequency spectrum. Low-frequency (30 KHz to 500KHz) tags have low system costs and are limited to short reading ranges.Low frequency tags may be used in security access and animalidentification applications for example. Ultra high-frequency (860 MHzto 960 MHz and 2.4 GHz to 2.5 GHz) tags offer increased read ranges andhigh reading speeds.

Embodiments of the RFID device are preferably implemented in conjunctionwith a Class-3 or higher Class IC chip, which typically contains theprocessing and control circuitry for most if not all tag operations.FIG. 2 depicts a circuit layout of a Class-3 IC 200 and the variouscontrol circuitry according to an illustrative embodiment forimplementation in an RFID tag. It should be kept in mind that thepresent invention can be implemented using any type of RFID device, andthe circuit 200 is presented as only one possible implementation.

The Class-3 IC of FIG. 2 can form the core of RFID chips appropriate formany applications such as identification of pallets, cartons,containers, vehicles, or anything where a range of more than 2-3 metersis desired. As shown, the chip 200 includes several circuits including apower generation and regulation circuit 202, a digital command decoderand control circuit 204, a sensor interface module 206, a C1G2 interfaceprotocol circuit 208, and a power source (battery) 210. A display drivermodule 212 can be added to drive a display.

A forward link AM decoder 216 uses a simplified phase-lock-looposcillator that requires an only a small amount of chip area.Preferably, the circuit 216 requires only a minimum string of referencepulses.

A backscatter modulator block 218 preferably increases the backscattermodulation depth to more than 50%.

A memory cell, e.g., EEPROM, is also present, and preferably has acapacity from several kilobytes to one megabyte or more. In oneembodiment, a pure, Fowler-Nordheim direct-tunneling-through-oxidemechanism 220 is present to reduce both the WRITE and ERASE currents toabout 2 μA/cell in the EEPROM memory array. Unlike any RFID tags builtto date, this permits reliable tag operation at maximum range even whenWRITE and ERASE operations are being performed. In other embodiments,the WRITE and ERASE currents may be higher or lower, depending on thetype of memory used and its requirements.

Preferably, the amount of memory available on the chip or otherwise isadequate to store data such that the external device need not be inactive communication with the remote device.

The module 200 may also incorporate a security encryption circuit 222for operating under one or more security schemes, secret handshakes withreaders, etc.

An external device can communicate directly with the chip by appendingan interface or “bus” 224 such as an industry-standard I²C or SPIinterface to the core chip. An illustrative bus 224 is a 2 pin I²Ccompliant bus coupled directly to the chip.

The RFID device may have a dedicated power supply, e.g. battery, maydraw power from a power source of the electronic device (e.g., battery,AC adapter, etc.), or both. Further, the RFID device may include asupplemental power source. Note that while the following descriptionrefers to a “supplemental” power source, the supplemental power sourcemay indeed be the sole device that captures energy from outside the tag,be it from solar, RF, kinetic, etc. energy.

FIG. 3 illustrates an RFID device 300 according to one embodiment of thepresent invention, here in the form of an RFID tag. As shown in FIG. 3,the tag 300 includes one or more antennae 304, a controller 308 coupledto a housing 302, power regulating circuitry 309, and a battery 310(rechargeable or not rechargeable) for providing power to the controlcircuitry 200. The controller 308 may be embodied in a chip, such aspart of the chip shown in FIG. 2.

An RF energy capture circuit, such as the power generation circuit 202of FIG. 2, generates power from incoming RF waves. This acts as a powersource for recharging the battery 310 (if applicable) and/or poweringvarious components of the tag.

The RFID device may also draw power from the power source of theelectronic device. For example, the communications interface may alsoprovide a mechanism for transferring power from the electronic devicepower source to the RFID device. Alternatively, a separate powerinterface may be provided as a means for transferring power from theelectronic device power source to the RFID device.

As shown in the embodiment of FIG. 3, one or more supplemental powersources 312-318 may be present (or present instead of the RF energycapture circuit) for recharging the battery 310 and/or powering the chip200. The supplemental power source(s) may be any type of supplementalpower source. Preferred supplemental power sources generate electricityfrom light (e.g., solar power), thermal energy, and/or kinetic energy.As shown in FIG. 3, illustrative supplemental power sources include aseismic transducer 312, piezoelectric transducer 314, acoustictransducer 316 and photovoltaic cells 318, each of which may alsocontain passive and/or low-power active circuits such as transformers,resistors and capacitors to condition the transducer outputs to matchthe input requirements of the recharging circuitry.

As FIG. 3 shows, seismic transducer 312, piezoelectric transducer 314,acoustic transducer 316 and/or photovoltaic cells 318 are used to eitherreceive energy from a generating source or transmit energy, and couplethat energy to the power regulator 309. All of the transducers, (312,314, 316, 318) are capable of generating and transmitting their ownenergy which will prove advantageous to the various embodiments in whichimplemented. Any one of the above mentioned devices or any combinationthereof may be used to present energy in the form of an alternatingcurrent (A.C.) or direct current (D.C.) voltage to power regulator 309.

To conserve power, the RFID device may enter a hibernation state duringat least some periods when the RFID device is not actively communicatingwith the electronic device or the remote device. In brief, variouscircuitry of the RFID device may be placed in a hibernation state duringperiods of inactivity to conserve power. A hibernation state may mean alow power state, or a no power state. Upon receiving a valid activationsignal or command, some or all of the circuitry may then be activated toperform some function.

In one aspect, the electronic device selectively wakes the RFID devicefrom a hibernation state, such as for uploading data to the RFID devicememory, checking the RFID device for updated data (e.g., to scan thememory, check whether a flag is set or not set, etc.), etc. Preferably,the circuitry is activated upon receiving an activation signal from theelectronic device and prior to receiving data from the electronicdevice.

Similarly, the remote device, e.g., reader may selectively wake the RFIDdevice from a hibernation state. For uploading data to the RFID devicememory, checking the RFID device for updated data (may scan the memory,or check whether a flag is set or not set), etc. Preferably, circuitryis activated upon receiving an activate command from the remote deviceand prior to sending the data to the remote device.

Any type of activation circuit, logic, or instruction set known in theart may be used to coordinate activation, or exit from a hibernationstate. With continued reference to FIG. 2, an illustrative batteryactivation circuit 214 is shown present on the illustrative chip 200.The battery activation circuit 214 remains active and processes incomingsignals to determine whether any of the signals contain an activatecommand or other signal indicating that the chip 200 should exit thehibernation state. If one signal does contain a valid activate command,additional portions of the chip 200 are wakened from the hibernationstate, and communication with the reader can commence. In oneembodiment, the battery activation circuit 214 includes anultra-low-power, narrow-bandwidth preamplifier with an ultra low powerstatic current drain. The battery activation circuit 214 also includes aself-clocking interrupt circuit and uses an innovative user-programmabledigital wake-up code. The battery activation circuit 214 draws lesspower during its sleeping state and is much better protected againstboth accidental and malicious false wake-up trigger events thatotherwise would lead to pre-mature exhaustion of the Class-3 tag battery210. While any type of battery activation circuit known in the art canbe potentially integrated into the system, an illustrative batteryactivation circuit 214 is described in copending U.S. patent applicationSer. No. 11/007,973 filed Dec. 8, 2004 with title “BATTERY ACTIVATIONCIRCUIT”, which is herein incorporated by reference.

A battery monitor 215 can be provided to monitor power usage in thedevice. The information collected can then be used to estimate a usefulremaining life of the battery.

As discussed above, standard RFID protocols are preferably used toenable communication between the remote device and the RFID device.Similarly, standard RFID protocols and/or variants thereof may be usedto enable communication between the RFID device and electronic device incommunication therewith. In such embodiments, the RFID device controllermay then not need additional circuitry to communicate in a secondprotocol. In other aspects, however, the RFID device may communicatewith the electronic device via another type of communication protocol,which may or may not require additional circuitry on the RFID device toenable communication via said protocol.

A basic RFID communication between an RFID device and a remote devicetypically begins with the remote device, e.g., reader, sending outsignals via radio wave to find a particular RFID device, e.g., tag viasingulation or any other method known in the art. The radio wave hitsthe RFID device, and the RFID device recognizes the remote device'ssignal and may respond thereto. Such response may include exiting ahibernation state, sending a reply, storing data, etc.

FIG. 5 depicts a method 500 for communication between an electronicdevice and a remote device according to one embodiment. In operation502, an RFID device receives data from a remote device via an airinterface using a RFID protocol. In operation 504, the data is stored inlocal memory on the RFID device. The data is sent to an electronicdevice via a direct physical connection in operation 506.

As an option, a flag may be set on the RFID device, indicating that datahas been written to the local memory of the RFID device. The electronicdevice may then merely check the flag to determine whether the RFIDdevice has data thereon. In one embodiment, the flag is set by setting asingle bit (or several bits) in memory to a particular state. Theelectronic device may then read the memory of the RFID device todetermine whether the flag is set or not. In another embodiment, theflat is set by periodically or continuously sending out a signal. In afurther embodiment, the flag is a mechanical mechanism. Once the data istransferred to the electronic device, the flag can be reset.

In one approach, communication between a remote device and an electronicdevice attached to the RFID device operates in a mailbox system. A pinon the bus can be programmed to bring out an internal signal used whenthe system is writing to internal memory of the RFID device. This can beused as the flag on the mailbox to tell the attached electronic devicethat the remote device has written into memory.

FIG. 6 depicts a method 600 for communication between an electronicdevice and a remote device according to one embodiment. In operation602, data is received by the RFID device from an electronic device via adirect physical connection. In operation 604, the data, is stored inlocal memory on the RFID device. A flag may be set. In operation 606, aquery from a remote device is received via an air interface, e.g., by RFsignal. In response to the query, the data is sent to the remote devicevia the air interface using an RFID protocol in operation 608.

Bidirectional communication between the remote device and electronicdevice is also possible, either in real time or via a mailbox system.Using a mailbox system, for example, the remote device and electronicdevice may use an agreed-upon area in the RFID device's memory to passmessages back and forth. The remote and electronic devices may eachperiodically check for a flag indicating that new data has been storedin the RFID device.

As alluded to above, implementations of the inventive system may includesensors. Such implementations typically involve transfer of data from asensor-equipped electronic device to a remote device, using the RFIDdevice as an intermediary. However, bidirectional communication is alsoanticipated, for such things as a manual request for a sensor reading,for retrieval of data stored on the electronic device as opposed to theRFID device, etc.

FIG. 7 illustrates an electronic device 103 with sensor capability andan optional display device. As shown, the electronic device 103 includesone or more sensors 702, 704 for detecting one or more environmentalconditions. The electronic device 103 may also include a visual displaydevice 706. An RFID device 102 is also coupled to the electronic device103.

The electronic device 103 can be attached to an item utilizing numerousmechanisms. For example, conventional mechanical fastening system, loopand hook-type arrangements, stitches, adhesives, as well as other knownfixation techniques may be employed to permanently or temporarily attachthe electronic device 103 to an item. The electronic device 103 can alsobe integrally formed with the item, or can be used as a stand alonedevice.

The sensors 702, 704 monitor the external environment in which theelectronic device 103 is operating. Virtually any environmentalcondition can be monitored. Illustrative sensors monitor temperature,humidity, Ph, sunlight, ultraviolet light, chemicals, radioactivity,pathogens, bacteria, viruses, prions, carbon dioxide level, etc. in theenvironment surrounding the tag 103.

One or more of the sensors can also monitor a condition, e.g.,characteristic or property, of the item to which attached, as opposed toan environmental condition. One example includes monitoring a surfacetemperature of the object to which attached.

The sensors 702, 704 may take readings continuously, or may takereadings at some interval, such as every 5 minutes, every hour, etc. Thereadings can be uploaded to the RFID device memory one at a time or inbatches, for later retrieval by the remote device.

The electronic device may enter hibernation state during the intervalperiod between readings to conserve battery power.

A controller 708 on the electronic device is coupled to the sensors 702,704 and may control the communication with the RFID device 102. Thecontroller 708 may include one or more silicon chips, programmablemicrocontrollers or discrete components.

In one example of operation, assume that a large number ofsensor-equipped electronic devices 103 are in the field of view of anRFID reader. Each electronic device 103 may be self energized (battery710 or otherwise) or powered via an external power supply, e.g., ACadapter. Each electronic device 103 is interfaced digitally to an RFIDchip and can activate the chip via a digital bus. The identity of eachRFID device 102 coupled to an associated electronic device 103 isascertainable via its EPC number In operation, the electronic device 103collects data, and periodically activates the RFID chip to store datasuch as sensor readings, time/date stamps, etc. in the RFID chip memory.When the RFID reader starts to collect sensor data, it activates eachRFID device 102 to read its memory to retrieve data stored by theassociated electronic device 103. The electronic device 103 may set aflag in the RFID chip memory, indicating presence of new data. In suchan approach, the reader may first query the flag bit of the RFID chipmemory.

The controller 708 may also set an alarm flag to indicate an alarmcondition. Alarm conditions can be based on exceeding levels orquantities, such as passing a threshold high temperature, falling belowa threshold low temperature, or detecting a certain number of viruses.Alarm conditions can also be based on historic data, such as a number ofhours that the carbon dioxide level was above a threshold.

Preferably the alarm flag is one or more bits stored in memory on theelectronic device or on the RFID device 102. A logic zero indicates anormal condition, while a logic one indicates an active alarm state (orvice versa). The flag preferably remains set until reset by anauthorized entity.

The visual display device 706 may output a reading of the sensor, anindication of the alarm condition, etc. For example, upon the electronicdevice 103 detecting an alarm condition based on sensor output, thevisual display device 706 is instructed or engaged to display a visualindicator of the alarm condition. For example, a visual indicator can bea simple color change, the placement of a symbol such as a circle ortriangle, or can include a number of textual or graphicalrepresentations. For instance, in one embodiment an internal or externaltemperature sensor can be employed so that in the event that theelectronic device 103 experiences a period below freezing, a visualindicator on the visual display device 706 will be on display to a user,who can quickly ascertain that an alarm condition has occurred.

As also alluded to above, implementations of the inventive system mayalso include displays. Such implementations may function in any modedescribed herein, and typically involve transfer of data from a remotedevice to a visual display device-equipped electronic device, using theRFID device as an intermediary. However, bidirectional communication isalso anticipated, for such things as sending confirmation back to theremote device that the data has been received, that the display has beensuccessfully updated, what the display is currently displaying, etc.

FIG. 8 depicts an electronic display device 103 with a visual displaydevice 802. An RFID device 102 is in electrical communication with theelectronic display device 103 via a direct physical connection. Datareceived by the RFID device 102 from a remote device is stored in thememory of the RFID device, the data being communicated to the electronicdisplay device via the direct physical connection.

The data may include display data (e.g., information or graphics to bedisplayed), a schedule for changing the display, etc. In one embodiment,the data correlates to a value stored in a pricing system database,e.g., a price of an item. In this way, the displayed data should alwayscorrespond to the price charged for an item at the point of sale device(e.g., cash register). In other embodiments, the data includespromotional material, advertisements, photographs, video, educationalmaterial, etc. Accordingly, virtually any type of information can bedisplayed.

In an illustrative mode of operation, data is received by the RFIDdevice 102 from the remote device via an air interface using an RFIDprotocol. The data is stored in local memory on the RFID device. Thedata is sent to the electronic display device 103 via the directphysical connection. The electronic display device 103 may store thedata in local memory of its own.

Preferably, the electronic display device provides a confirmation thatit has successfully received the data. Such a confirmation may be storedin the RFID device memory, for transmission to the remote device uponreceiving a query therefrom.

As in other embodiments described herein, a flag may be set, indicatingthat data has been written to the RFID device memory. Setting the flagmay include sending an electronic signal detectable by the electronicdisplay device via the direct physical connection. For example, a signalmay be sent to a pin of the bus, which the electronic display devicedetects. The electronic display device then knows to retrieve theupdated data from the RFID device memory.

In another approach, setting the flag includes setting the state of abit or bits in the RFID device memory. The electronic display device maythen periodically poll the memory to determine whether the flag is setor not.

Whichever flag implementation is used, the electronic display device mayperiodically check the status of the flag, or it may continuously checkthe status of the flag. Upon transferring the data to the electronicdisplay device, the flag may be reset.

The visual display device 802 can be any type of display device.Illustrative visual display devices include color-changing strips,electrophoretic displays, electrokinetic displays, light emitting diodes(LEDs), liquid crystal displays (LCDs), backlit displays, etc. Onepracticing the invention will understand that the type of visual displaydevice 802 used may depend on the power supply to the electronic displaydevice. If the electronic display device has a virtually unlimited powersupply, e.g., is coupled to an AC adapter, then visual display devicesusing higher power may be used. If the electronic display device has alimited power supply, e.g., onboard battery, low power visual displaydevices such as LCDs and state changing electrochemical strips arepreferred.

In some low-power embodiments, the visual display device 802 may be anultra-low power display device, so as to use as little battery power aspossible and thereby maximize the active life of the electronic displaydevice. The visual display device 802 in such embodiments is preferablyone that continues to display the visual indicator even after power tothe display device is removed. This minimized power consumption, whichis of particular importance for devices of limited battery life. Oneillustrative visual display device 802 includes a chemical strip thatchanges color upon receiving an electrical signal from a controller. Thecolor change is permanent until reset electronically by the controller,and does not require a continuous electric current to maintain the colorchange.

The display media of the visual display device 802 can also be abi-stable, non-volatile display medium. Examples of bi-stablenon-volatile mediums include but are not limited to encapsulated andun-encapsulated electrophoretic material, Cholesteric materials, polymerdispersed cholesteric liquid crystals (PDChLC), encapsulated cholestericmaterials, separated redox and dye reaction materials such as DowCommotion® display medium, ph sensitive dyes, electrothermochromicsdisplays and thermo-chromic, zenithal bi-stable, nematic, and surfacestabilized ferroelectric liquid crystals.

The display media can further comprise electronic ink, wherein theelectronic ink is capable of displaying a graphical representation onthe visual display device 802. The term “electronic ink” as used hereinis intended to include any suitable bi-stable, non-volatile material.The term “bi-stable” as used herein is intended to indicate that theparticles of the imaging material can alternately occupy two stablestates. For example, the particles corresponding to different pixellocations of the display assembly can alternately occupy an ON or an OFFstate to form selected indicia.

FIG. 9 depicts a method 900 for updating an RFID-based display accordingto one embodiment. Such a method may be executed by a computer system,handheld system (e.g., portable RFID reader), etc. In operation 902,identification of an item is received. For example, a list of items canbe presented to a user on a graphical user interface, upon which theuser selects a name of the item from the list. Further, the item may beidentified based on a scan of a bar code associated with the item. Forinstance, a handheld RFID reader with integral barcode scanner can scana barcode on the item. Additionally, the item may be identified based onan RFID-based code retrieved from an RFID tag associated with the item.For instance, a handheld RFID reader can scan the RFID tag coupled tothe item to retrieve its EPC number, then correlate that EPC number tothe item's identification as stored in a database.

In operation 904, updated display information relating to the item isreceived. This may include manual user input of alphanumeric characters,selection of graphics or video, etc.

In operation 906, the updated display information is sent to an RFIDdevice via an air interface, for transfer to an electronic displaydevice coupled to it.

In optional operation 908, a confirmation may be received from the RFIDdevice that the data has been received and stored in a memory thereof.

In optional operation 910, a confirmation may be received from the RFIDdevice that the data has been successfully transferred to the electronicdisplay device. Such confirmation may initiate with the RFID device, orthe electronic, display device.

In any of the embodiments presented herein, the display data can beupdated periodically, or at will. Further, the display can time out andthen change or revert to another state. For instance, the price shown ona display can be changed for a “one hour sale.” Because the centralpricing system may query the RFID device for the current state of theelectronic display device, the new price would be charged at the pointof sale.

Various implementations of the inventive system may include batterymonitoring capability. Accordingly, an RFID system according to oneembodiment of the present invention includes an electronic device thatis powered by a battery (“battery” includes one or more batteries,disposable and rechargeable batteries, etc.), an RFID device inelectrical communication with the electronic device, and a mechanism forestimating a remaining potential energy of the battery. A flag is set onthe RFID device when an estimated remaining potential energy of thebattery is below a predefined threshold. The flag may be a single bit,or several bits, set to a state indicative of whether an estimatedremaining potential energy of the battery is below a predefinedthreshold. A remote device may then query the RFID device to determinewhether the flag is set. If the flag indicates that the remainingpotential energy of the battery is below a predefined threshold, amessage may be output to a user, e.g., to indicate that it may be timeto change the battery, recharge the battery, etc.

In another embodiment, the RFID device stores an indication of acondition of the battery powering the electronic device. Such indicationmay be a flag as above, but may also include an indication of actualusage, an estimated life remaining in the battery, whether the batteryis dead, an output level of the battery, etc.

Any type of known circuitry for monitoring battery usage may beimplemented. For instance, an illustrative battery monitoring circuitmay keep track of an amount of time the electronic device is active,using the resulting time count to estimate whether the remainingpotential energy of the battery is above or below a predeterminedthreshold. Illustrative battery monitoring circuits are presented belowwith reference to FIGS. 10-12. Another type of circuit may monitor apower level produced by the battery to estimate whether the remainingpotential energy of the battery is above or below a predeterminedthreshold.

The circuitry for monitoring usage of the battery may be present on theRFID device. This may require that at least a portion of the RFID devicebe activated while the electronic device is active. The circuitry formonitoring usage of the battery may also be present on the electronicdevice. Thus, while the illustrative circuits shown in FIGS. 10-12 aredescribed as being present on the RFID device, it should be understoodthat the circuits may be implemented on the electronic device. Further,portions of the circuits may be divided across, present on, or shared byboth devices. Accordingly, the flag may be set by the RFID device or theelectronic device.

Again, while the illustrative systems shown in FIGS. 10-12 are describedas being present on the RFID device, it should be understood that thesystems may be implemented on the electronic device with minormodifications and without undue experimentation, as will be understoodby those skilled in the art. For example, instead of counting while theRFID chip is active, a system might count while the controller of theelectronic device is active. Similarly, instead of sending the countdirectly to a remote device, a system might send it to the RFID device.

FIG. 10 illustrates one embodiment of a system 1000 having an RFID tagbattery monitor 1002 in which a mechanism, e.g., dedicated oscillator1004, tracks the total amount of time that the tag has spent in itshigh-power “Activated” state. Another mechanism, e.g., counter 1006,generates a value based on the tracking, where the value can thereby beused to estimate the total power consumed by the tag. A remote devicesuch as a reader (see FIG. 1) can query the state of this counter 1006to accurately determine how much of the battery energy has been consumedby the tag, how much life is left in the battery and/or how many moreoperations can be performed before the tag's battery is exhausted.

In a simple embodiment, the battery monitor oscillator 1004 operatesonly when the tag is active. In this circuit 1000, the battery monitoroscillator 1004 is, using a frequency divider 1012, divided down by theratio of 16:1 from the tag's internal oscillator 1008 (typically a 40KHz oscillator that might be used in the C1G2 specification) that thetag uses to parse and decode data it receives from the reader. In thisway, the battery monitor oscillator 1004 operates accurately at 2.5 KHzsince this frequency is ultimately derived from the reader's oscillator1010 or accurately generated on the RFID chip to an accuracy of betterthan ±15%. If the tag is commanded by the reader to download data at afaster rate than 40 Kbps, then the tag may increase the divider ratiofor the battery monitor 1002 above 16:1 (as shown in FIG. 10) to makesure the battery monitor oscillator 1004 continues to run at 2.5 KHz. Inthis way, the battery monitor oscillator 1004 can provide a consistenttracking of the battery usage, regardless of data speed.

The battery monitor oscillator 1004 drives the “Battery Monitor Counter”1006 that increments when the oscillator 1004 is running and continuesto store the current cumulative count during periods when the tag isinactive or unpowered. The contents of the battery monitor counter 1006can be read in a pre-assigned location of the optional user memory bank1014 of the tag. In this example, the oscillator 1004 drives the counter1006 at 2.5 KHz. The counter 1006 is preferably at least 32-bits long tomake sure it never overruns its storage limit, but only the state of the16 MSBs have to be addressable by the reader.

The divider 1012 and the resulting slow 2.5 KHz oscillation frequency ofthe battery monitor oscillator 1004 ensure that the power dissipation ofthe battery monitor circuit 1002 is negligible compared to the rest ofthe active power dissipation of the tag. Typical power dissipation willbe only a few nanoamperes.

Those skilled in the art will understand that the simple digitalfrequency divider 1012 may be replaced with current mirrors andreference current sources. In a similar manner, the battery monitor 1002need not operate at a fixed frequency but may be made to vary inproportion with the variations in the power dissipation of the RFID chipassuming that the power dissipation may vary as a function of theforward data rate, whether the tag is writing data to memory or not,whether or not the tag is operating a sensor, etc.

In a variation on the above, a second oscillator can be run during othertag states to estimate off-time usage. Note that this may require asecond register to store the counts from the second oscillator. Then thereader can query both registers and estimate the remaining battery lifeusing both active and inactive times.

Continuous Battery Monitoring with Analog Control:

In a more complex illustrative embodiment shown in FIG. 11, a tagbattery monitor 1100 may also include a fixed-frequency ultra-low-poweroscillator 1102 with multiple or variable frequency dividers that candivide the fixed-frequency output by varying amounts depending onwhether or not the tag is in a higher-power activate state, a low-powerinactive state, or other power state. As in the circuit 1002 of FIG. 10,the battery monitor circuit 1100 of FIG. 11 is designed to consume onlya small fraction of the power of the chip itself so as not tosignificantly shorten the battery life of the tag.

In this illustrative embodiment, assume the RFID tag contains atemperature sensor and supports 3 different operating modes:

-   -   a hibernation or “inactive” mode in which it consumes only 0.1        μA in power;    -   an “activated” mode in which it exchanges data with the reader        and consumes 10 μA    -   a “sense” mode in which it takes and records a temperature        reading and consumes 30 μA.

While the amount of time the tag will spend in each of these modesvaries widely, the circuit 1100 monitors the time spent in each mode andaccumulates the effect this will have on battery life.

The circuit shown in FIG. 11 contains a precision calibratedUltra-Low-Power (“ULP”) oscillator 1102 that runs at a frequency rate ofonly about 500 Hz and consumes only about 3 nA of power. This oscillator1102 can run continuously which permits more accurate measurement ofpower consumption since it now monitors and measures inactive“hibernate” power consumption in addition to the active powerconsumption measured in FIG. 10. Addition of the ULP oscillator 1102also facilitates other important tag functions like enabling a real timeclock, enabling logging of temperature and other sensor data, etc.

The ULP oscillator 1102 in this embodiment consists of a VoltageControlled Oscillator (“VCO”) formed with three inverters (I1, I2,I3)connected with feedback to form a ring oscillator. The frequency of thisoscillator is controlled by matched pairs of current mirroredtransistors P5/N5, P6/N6, P7/N7, and a capacitor connected to the outputof I1. The current flowing in these transistors is in turn controlledboth by the “2 nA reference current” flowing in P3 and the analogcurrent multiplier circuit formed by N1, N2, N3, and N4.

In the lowest-power “Inactive” mode, the tag is neither in the “Sense”nor “Active” state, and the negative-sense inputs “NSense” and “NActive”are both high. This effectively connects transistors N1, N2, and N3 inparallel at the drain of N3 for a total conductivity of 1×+2×+297×=300×.The “1×” or “297×” here refers to the relative “size”, “conductivity”,or more accurately “g_(m)” of the transistors. Following standardcurrent-mirror design practice, the effective mirror ratio is now30×/300× which sets the currents flowing through P4 and N4 (plus eachthe other current mirrored transistor pairs P5/N5, P6/N6, and F7/N7) at0.2 nA. An additional 2 pf of low-leakage non-junction capacitance isalso added to the output of inverter I1 to reduce the oscillatorfrequency to about 500 Hz. Note that, in this “inactive” mode the powerdissipation of this entire battery monitoring circuit totals only about5 nA, which is much less than the power consumed by the tag itself.

However, when the tag enters the “active” state, transistor N2 isdisconnected from the mirror circuit. This changes the currentmultiplier ratios as follows: the total conductivity is now 3× (1×+2×),the ratio is now 30×/3×, the P4 current has increased to 20 nA, and theVCO oscillator frequency has increased from 500 Hz to 50 KHz. Note thatwhen the tag power dissipation increased 100× from 0.1 μA to 10 μA, thebattery monitor circuit responded by increasing the counter frequency by100× also. Also, note that although the monitor power dissipationincreased to about 0.1 μA, it remains less than 1% of the total powerdissipation of the chip itself.

Finally, when the tag enters the highest-power “sensor” state,transistors N2 and N3 are both disconnected from the mirror circuit.This changes the current multiplier ratios as follows: the totalconductivity is now only 1×, the ratio is now 30×/1×, the P4 current hasincreased to 60 nA, and the VCO oscillator frequency has increased to150 KHz. Note that when the tag power dissipation increased 300× from0.1 μA to 30 μA, the battery monitor circuit responded by increasing thecounter frequency by 300× also. Also, note that although the monitorpower dissipation increased to about 0.3 μA, it remains less than 1% ofthe total power dissipation of the chip itself. It is well known tothose skilled in the state of the art that well-designed VCO oscillatorslike the one shown in FIG. 11 or described in U.S. Pat. No. 4,236,199,can be accurately controlled over frequency ranges of 10,000:1 or more.

A Calibrated Ultra-Low-Power Current Source:

While the preceding discussion shows how the battery monitor circuitwill work with a 2 nA reference current source, no such current sourceshave ever existed in the IC chip world. For example, just trying toscale a conventional PMOS transistor to source only this much current(with it's gate grounded and it's source at 1.2V) would require thechannel, length to be scaled to over 100,000 microns—hardly a practicaldesign. And in any case, the accuracy and stability of any 2 nA currentsource would be extremely poor without a method for accuratelycalibrating this current. FIG. 11 therefore includes a practical circuitfor generating and calibrating the 2 nA current-source. Again, it shouldbe stressed that the 2 nA current source specification is by way ofexample only, and higher and lower currents can be used with thecircuit.

In FIG. 11, the calibrated current source is controlled by a replica ofthe first VCO that is used to drive the accumulating counter 1104,except that this reference VCO 1106 runs at a constant frequencyindependently of the Activate or Sense modes of operation. In thisexample the VCO 1106 operates at 5000 Hz, but again, it could operate ateither a higher or lower frequency. The output of the reference VCO 1106is buffered and clipped to form a square wave and used to drive P1 andP2. P1, P2 and their associated capacitors form a “switched capacitor”precision resistor 1108. The bias current flowing through this networkis nominally: I=(C1)×(f)×(ΔV)=20 ff×500 Hz×0.5V=50 pA. Nominally, this50 pA bias current also flows through the calibration matrix and inducesan offset voltage across the calibration matrix of 0.75V—assuming that 3of the bypass calibration transistors are turned off. With a nominal Pthreshold voltage of 0.44V, then both P3 and P13 will be biased atexactly 0.01 V above their thresholds and will in theory each injectexactly 2 nA into the circuit

In practice P3 and P13 (and the other mirrored transistors in thecircuit) can operate either just above or below their respectivethreshold voltages, i.e. both can be operated in their sub-thresholdregion if necessary to keep the reference current low. Also in practicenone of these nominal variables are well controlled, so withoutcalibration, the resulting reference currents and oscillationfrequencies might vary greatly from their nominal values due tovariations in threshold voltage, temperature, and sub-thresholdcharacteristics of both the diodes and transistors.

Accurate calibration of the current source and the oscillator frequencyis therefore achieved as follows. The reader issues a “Calibration”command and sends a 5000 Hz reference tone to the tag The tag uses asimple PLL circuit 1112 to compare the reader reference frequency withit's own reference oscillator frequency and adjusts the 5 digital inputsto the calibration matrix as necessary to force it's own referenceoscillator frequency to match that of the reader. Once set, the digitalcalibration settings are permanently stored in memory, e.g., eitherEEPROM or static RAM until a reader tells the tag to re-calibrateitself.

The calibration matrix shown in FIG. 11 is digitally adjustable. Thenominal maximum voltage across the calibration matrices is 1.25 V (250mV forward bias at 50 pA for each “1×-sized” diode). Coarse adjustmentsof about 250 mV are made by shorting out completely one or more of the“Cal” diodes. As shown in FIG. 12, the circuit of FIG. 11 can beextended as necessary to provide digital adjustments as fine as 1 mV.

Continuous Battery Monitoring with Digital Control:

FIG. 12 shows another embodiment 1200 of the invention including asingle calibrated ULP oscillator 1202 running at 1 KHz and two digitaldividers 1204, 1206 to monitor time in each of the three illustrativetag power modes (Inactive, Active, Sensor) and to total the cumulativeimpact of operating in each of these modes on the remaining batterycapacity. When operating in the “Inactive” mode the 1 KHz oscillatoroutput is divided 300:1 before the output is fed into the 40-bitaccumulating counter 1208. In the “Active” mode the reference oscillatoris divided 3:1, and in the “Sensor” mode it passes directly to theaccumulating counter 1208. In this circuit 1200, changes to thereference oscillator frequency are minimized, and a constant 10 pA biascurrent passes through P1 and P2 to the calibration matrix.

Auto-Calibration:

The calibration matrix shown in FIG. 12 is digitally adjustable with acombination of both fine and coarse adjustment bits. In this example,use of an ultra-low-bias current of only about 10 pA reduces the offsetvoltage across the parallel combinations of the four diodes (a,b,c,dwhich total 22.1× in size) to only about 200 mV. In one preferredembodiment, the tag will calibrate itself with the following simplifiedalgorithm.

Initially all 24 of the calibration transistors are turned on and thecalibration matrix is shorted out completely. The tag referencefrequency will then initially exceed that of the the reader referencefrequency and this fact is detected by the PLL oscillator 1212 shown inFIG. 12. In response, the calibration logic starts turning off each ofthe N1 a, N2 a, N3 a . . . transistors in sequence until the PLL detectsthat the tag frequency has dropped below that of the reader referenceoscillator (or until N1 a through N6 a are all off). Each disconnected“a-series” transistor increases the voltage to the gate of P3 by 200 mV.If and when the PLL detects that the tag frequency has dropped below thereader reference frequency of 1000 Hz, the calibration circuit turns thelast two “a-series” transistors that it had switched off, back on. The“a-series” coarse calibration sequence is now complete.

Next, the tag begins the “b-series” calibration sequence by turning offeach of the N1 b, N2 b . . . transistors in sequence until the PLLdetects that the tag frequency has dropped below that of the readerreference oscillator (or until N1 b through N6 b are all off). Eachdisconnected “b-series” transistor decreases the size of the diode from22.18× to 2.18×, and this increases the voltage to the gate of P3 by 60mV. This is because the forward-current/junction-area of the diode is anexponential function of the forward voltage with a slope of about 60mV/decade at room temperature. If and when the PLL detects that the tagfrequency has dropped below the reader reference frequency of 1000 Hz,the calibration circuit turns the last two “b-series” transistors thatit had switched off, back on. The “b-series” calibration sequence is nowcomplete.

Next, the tag begins the “c-series” calibration sequence by turning offeach of the N1 c, N2 c . . . transistors in sequence until the PLLdetects that the tag frequency has dropped below that of the readerreference oscillator (or until N1 c through N6 c are all off). Eachdisconnected “c-series” transistor decreases the size of the diode from2.18× to 1.18×, and this increases the voltage to the gate of P3 by 20mV, based on the equation ΔV=(log₁₀2.1/1.18)(60 mv/decade)=20 mV. Asbefore, if and when the PLL detects that the tag frequency has droppedbelow the reader reference frequency of 1000 Hz, the calibration circuitturns the last “c-series” transistor that it had switched off, back on.The “c-series” calibration sequence is now complete.

Finally, the tag begins the “d-series” calibration sequence by turningoff each of the N1 d, N2 d . . . transistors in sequence until the PLLdetects that the tag frequency has dropped below that of the readerreference oscillator. Each disconnected “d-series” transistor decreasesthe size of the diode from 1.18× to 1.0×, and this increases the voltageto the gate of P3 by 5 mV, based on the following equation:

ΔV=(log₁₀1.1/1.0)(60 mv/decade)=5 mV

The calibration circuit then stops and locks the digital inputs to eachof the calibration transistors in either EEPROM or static memory untilthe tag receives another “Calibration” command from the reader. The fullauto-calibration sequence is now complete.

If necessary, even finer adjustments could be made by connecting evenmore diodes of different sizes in parallel thereby controlling theforward drop by increments as small as 1 mV. The net effect is to adjustthe voltage across the calibration matrix such that at the nominal 10 pAbias current, there is just the right combination of diodes so that theinput voltage to P3 is exactly what is necessary to produce the 2 nAreference current. The negative feedback employed during the calibrationsequence ensures the the tag will calibrate itself accurately despitethe variability in threshold voltages, leakage currents, etc.

Once calibrated, the bias voltage on the gate of P3 is maintained bynegative feedback through P1 and P2. If, for example, the P3 gatevoltage were to decrease, then the current through P3 would increase andthe reference frequency would also increase. This would increase thecurrent flowing through P1 and P2 which would fully restore the P3 gatevoltage to the original value set by the calibration sequence.

While circuits like those shown in FIGS. 11 and 12 can achieve initialfrequency and current calibration accuracies of better than ±10%, thisaccuracy may be degraded by changes in temperature or by subsequentvariations in the power supply voltage. The best results are achieved byminimizing the variability of the power supply voltage using either aband-gap regulator or a battery power supply. In addition, the regulatedpower supply could compensate for both the temperature effect on thethreshold voltage of P3/P13 and for the 2 mV/degree variation of thecalibration diodes. If necessary, the accuracy can also be furtherimproved by periodic re-calibration of the tag.

Additional Considerations:

To estimate the remaining life of the battery, the reader can query thetag for the value stored in the counter (or derivative of the value) andcompare that value (or derivative thereof) to a benchmark valuerepresenting tag life or battery life. For instance, the reader can usean experimental average life count, of, say 7 million, and compare it tothe value in the counter to estimate power usage and remaining batterylife. Alternatively, the comparison may be based on some predefinedthreshold value, where the count is compared to the threshold value todetermine whether the battery is at a predefined stage in its life.

If the user changes the tag battery, the count can be reset from thereader. Or the count can automatically reset if a battery is removed andreplaced. The reader can send an alert by email, integrated displayscreen, computer link, etc. if a tag life is nearing its end. Similarly,the tag can include an integrated display screen, activated at userrequest, to show an estimated power consumption/life remaining.

Note that the methods and circuits herein could also apply to measuringthe service life of the tag or electronic device, measuring tag orelectronic device activity, estimating a remaining useful life of thetag or electronic device (e.g., if the circuitry is prone to a limitedlifetime), etc.

Many types of devices can take advantage of the embodiments disclosedherein, including but not limited to RFID systems and other wirelessdevices/systems. To provide a context, and to aid in understanding theembodiments of the invention, much of the present description shall bepresented in terms of an RFID system such as that shown in FIG. 1. Itshould be kept in mind that this has been done by way of example only,and the invention is not to be limited to RFID systems, as one skilledin the art will appreciate how to implement the teachings herein intoelectronics devices in hardware and/or software. In other words, theinvention can be implemented entirely in hardware, entirely in software,or a combination of the two. Examples of hardware include ApplicationSpecific Integrated Circuits (ASICs), printed circuits, monolithiccircuits, reconfigurable hardware such as Field Programmable Gate Arrays(FPGAs), etc. The invention can also be provided in the form of acomputer program product comprising a computer readable medium havingcomputer code thereon. A computer readable medium can include any mediumcapable of storing computer code thereon for use by a computer,including optical media such as read only and writeable CD and DVD,magnetic memory, semiconductor memory (e.g., FLASH memory and otherportable memory cards, etc.), etc. Further, such software can bedownloadable or otherwise transferable from one computing device toanother via network, wireless link, nonvolatile memory device, etc.

One skilled in the art will appreciate how the systems and methodspresented herein can be applied to a plethora of scenarios and venues,including but not limited to automotive yards, warehouses, constructionyards, retail stores, boxcars and trailers, etc. Accordingly, it shouldbe understood that the systems and methods disclosed herein may be usedwith objects of any type and quantity.

Illustrative Embodiment

Following is a nonlimiting example of a system and its operationaccording to one embodiment of the present invention. The embodimentdescribed is meant to illustrate one possible implementation of theteachings provided herein, and in no way should be construed aslimiting.

An RFID chip is coupled to an electronic device via an I2C bus. The RFIDchip contains 8 KB of memory. All memory accesses are on 16-bit wordboundaries. 512 bytes are used internally by the chip, leading 7.5 KB ofuser memory. Memory is addressed normally through the I2C interface inthat address zero accesses the first word in memory. Through the airprotocol, however, the most significant eight bits of the word addressare inverted before accessing memory so that air protocol address zeroaccesses physical address 0xff0. The 512 bytes used internally by thechip should be avoided. These 512 bytes are mapped into addresses 0x000to 0x0ff for the I2C interface and addresses 0xf00 to 0xfff for the airinterface.

FIGS. 13A-C illustrates an exemplary Ultra-thin Quad Flat No-Lead (UQFN)package 1300 that may be used in conjunction with this illustrativeembodiment. As shown, the package has 16 pins, some of which areindicated by numerals 1, 4, 8, 12, 16 in circles. An illustrative pinout map is presented below.

Pin Out

Pin Name Dir Description 14 VDD3V in 3 volt power supply pin 6 VSS inGround 4 test_n in Test enable - Active Low A low on this pin enablesI2C communication 16 e_clk in External Clock Nominal 2.88 MHz. Clockused when in I2C communication mode. This can be a lower frequency butmust be at least 5 times the I2C clock speed. 15 probe_d out DigitalProbe This pin can be programmed to bring out internal digital signals.Order the part with this pin set to bring out the ee_wr_n signal. 2 I biI2C data 1 c in I2C clock Maximum 720 kHz.

I2C Communication

Communication through the I2C interface is command response. Theattached electronic device writes a command to the chip and then pollsusing reads looking for the status flag to go high. I2C communication isenabled by pulling the test_n pin low and supplying a clock to the e_clkpin. The chip must be powered at the time, e.g., through the VDD3V pin.

Data Format

The chip expects data in a certain format. The format includes the I2Caddress, 16 bits reserved for future use, the data, the memory address,and the command. All commands may send all fields even if the field isnot needed for the particular command. The chip follows the standard I2Cformat of acknowledging every 8 bits of a transmission. First the 7 bitI2C address is sent. This is fixed at I2C address 0x27. The read/writebit follows the I2C address after which the chip will acknowledge thosebits by pulling the I pin low for one C clock cycle. The read/write bitis always write for a command and always read when looking for thecommand reply.

All I2C communication is most significant bit first for a given field.The Bits column in the following tables is in WIRE order.

Command Data Format

Bits Description 0–7 RFU  8–15 RFU 16–23 Data least significant byte Bit16 (the 16^(th) bit sent after the address and read/write bits) is themost significant bit of the least significant byte of the 16 bit dataword. 24–31 Data most significant byte 32–39 Memory address leastsignificant byte 40–43 Command code (sent together with bits 44–47 inone 8 bit group) 0 - NOP 1 - Write Memory Word 2 - Read Memory Word 3 -0xF RFU and factory only 44–47 Memory address most significant nibble.(sent together with bits 40–43 in one 8 bit group)

Reply Data Summary

Bits Description 0 Status Flag 0 - requested command not complete 1 -requested command complete and data (if any) is valid 1–7 RFU  8–15 RFU16–23 Data most significant byte 24–31 Data least significant byte

Handshaking with a Reader Simple Reader to Tag Protocol

If all that is required is for the reader to be able to write a fixedamount of data to the electronic device, the following steps may beused:

-   -   1. The electronic device monitors the probe_d pin and counts the        number of words being written. There will be one low-going pulse        per 16 bit write.    -   2. The reader writes the data to a known place in the tag        memory, for example starting at address 0x000.    -   3. Once the electronic device sees the correct number of pulses        on the probe_d pin, it places the tag in I2C communication mode        by setting test_n low and supplying a clock to the e_clk pin.    -   4. The electronic device sends the correct number of read        commands through the I2C bus starting at address 0xff0 gathering        the expected number of words. Keep in mind that the eight most        significant bits of the address may need to be decremented when        crossing from address 0xfff to address 0xfe0.    -   5. It then returns the tag to normal operation mode by setting        test_n high.

Reader to Tag Protocol with an Arbitrary Number of Word Transfers

-   -   1. The electronic device monitors the probe_d pin. There will be        one low-going pulse per 16 bit write.    -   2. The reader writes the number of words it wants to transfer to        a predefined place in the tag memory for example address 0x000.    -   3. The electronic device then places the tag in I2C        communication mode by setting test_n low and supplying a clock        to the e_clk pin.    -   4. The electronic device sends a read command through the I2C        bus, reading address 0xff0. This is the expected number of words        the reader wants to transfer.    -   5. The electronic device writes a zero to location 0xff0 and        returns the tag to normal operation by setting test_n high.    -   6. The reader meanwhile reads memory address 0x000 in the tag        until it returns zero.    -   7. The reader then writes the number of words it wishes to        transfer into a known place in the tag's memory for example        address 0x010.    -   8. Once the electronic device sees the correct number of pulses        on the probe_d pin, it places the tag in I2C communication mode        by setting test_n low and supplying a clock to the e_clk pin.    -   9. The electronic device sends the correct number of read        commands through the I2C bus starting at address 0xfe0 gathering        the expected number of words.    -   10. It then returns the tag to normal operation mode by setting        test_n high.

Tag to Reader Protocol

For tag to reader communication, the reader must periodically poll thetag, reading a predefined location in the tag's memory. The tag writesits data to a known place in memory. It then writes the number of wordsit wishes to transfer into the predefined location. Once the reader seesa non-zero value in the predefined location, it reads the number of datawords from the known location. The reader then writes a zero to thatlocation. This write operation flags the electronic device monitoringthe probe_d pin that the reader has received the data.

Bidirectional Protocol

By combining protocols in the sections above entitled “Simple Reader toTag Protocol” and “Reader to Tag Protocol with an Arbitrary Number ofWord Transfers”, but using different addresses for the predefinedlocations in each direction, a bidirectional protocol and be achieved.

While various embodiments have been described above, it should beunderstood that they have been presented by way of example only, and notlimitation. Thus, the breadth and scope of a preferred embodiment shouldnot be limited by any of the above-described exemplary embodiments, butshould be defined only in accordance with the following claims and theirequivalents.

1. A Radio Frequency Identification (RFID) system, comprising: anelectronic device being powered by a battery; an RFID device inelectrical communication with the electronic device; and a mechanism forestimating a remaining potential energy of the battery, wherein a flagis set on the RFID device when an estimated remaining potential energyof the battery is below a predefined threshold.
 2. A system as recitedin claim 1, wherein the RFID device includes circuitry for monitoringusage of the battery.
 3. A system as recited in claim 1, wherein aremote device queries the RFID device to determine whether the flag isset.
 4. A system as recited in claim 1, wherein the electronic deviceincludes circuitry for monitoring usage of the battery, the electronicdevice setting the flag on the RFID device.
 5. A system as recited inclaim 1, wherein the electronic device includes a visual display device.6. A system as recited in claim 1, wherein the electronic deviceincludes a sensor.
 7. A Radio Frequency Identification (RFID) system,comprising: an electronic device being powered by a battery; and an RFIDdevice in electrical communication with the electronic device, whereinthe RFID device stores an indication of a condition of the batterypowering the electronic device.
 8. A system as recited in claim 7,wherein the RFID device includes circuitry for monitoring usage of thebattery.
 9. A system as recited in claim 7, wherein a flag is set on theRFID device when the condition of the battery is such that an estimatedremaining potential energy thereof is below a predefined threshold. 10.A system as recited in claim 9, wherein a remote device queries the RFIDdevice to determine whether the flag is set.
 11. A system as recited inclaim 7, wherein the electronic device includes circuitry for monitoringusage of the battery, the electronic device setting a flag on the RFIDdevice when the condition of the battery is such that an estimatedremaining potential energy thereof is below a predefined threshold. 12.A system as recited in claim 7, wherein the electronic device includes avisual display device.
 13. A system as recited in claim 7, wherein theelectronic device includes a sensor.
 14. A Radio FrequencyIdentification (RFID) device, comprising: an interface for providing adirect physical connection to an electronic device that is powered by abattery; a memory for storing an indication of a condition of thebattery powering the electronic device; and circuitry for sending theindication stored in the memory to a remote device via an air interface.15. A device as recited in claim 14, wherein the indication is a singlebit set to a state indicative of whether an estimated remainingpotential energy of the battery is below a predefined threshold.
 16. Amethod for indicating a condition of a battery, comprising: estimating aremaining potential energy of a battery coupled to an electronic device;setting a flag on an RFID device when the estimated remaining potentialenergy of the battery is below a predefined threshold; and sending astate of the flag to a remote device via an air interface.
 17. A methodas recited in claim 16, wherein the RFID device is in communication withthe electronic device via a direct physical connection.
 18. A method asrecited in claim 17, wherein setting the flag includes setting the stateof a single bit or series of bits in memory.
 19. A method as recited inclaim 16, wherein the RFID device includes circuitry for monitoringusage of the battery.
 20. A method as recited in claim 16, wherein aflag is set on the RFID device when the condition of the battery is suchthat an estimated remaining potential energy thereof is below apredefined threshold.
 21. A method as recited in claim 20, wherein aremote device queries the RFID device to determine whether the flag isset.
 22. A method as recited in claim 16, wherein the electronic deviceincludes circuitry for monitoring usage of the battery, the electronicdevice setting a flag on the RFID device when the condition of thebattery is such that an estimated remaining potential energy thereof isbelow a predefined threshold.
 23. A method as recited in claim 16,wherein the electronic device includes a visual display device.
 24. Amethod as recited in claim 16, wherein the electronic device includes asensor.